Hi ​@jessta, welcome to the community.  

So I can’t explain why re-uploading the exported doesn’t map to the original columns, but I suspect there may be some inconsistent with the way either your system or how Excel saves/stores CSV - apparently it depends on whether its saved in UTF-8 or other encoding (might be shown as ANSI, UTF-8, etc) which can corrupt or misalign columns.  Or another common error I find is there are special characters (typical in multi-lingual names) that corrupts the data.   

Frankly, I recommend using something more consistent/stable such as Google Sheets which I typically get less formatting issues.

However, one workaround that is super quick if you simply want to divide a list in half, is to use the “Sample List” feature.  What this does is, you can ask Klaviyo to randomly select X users into a new List.

This is in the upper right section of any List (or Segment) under the “Manage List” option:

When you click it, it will prompt you to select your Sample List size:

In your case, you can just take the number of people in your List and divide it by 2.  So if your list is 10,000 people, just input in a Sample size of 5000.  That will give you a new List that’s half the size (e.g. 5000).  

If you need the other half as a List, then create a new Segment of people that are in the main List, but not in the new Sample List.  That should pull out anyone that’s not in the random sample List. (You can convert the Segment into a List if you need it as a List).

It’s sort of a strange workaround, but it’s sometimes faster to do this then to deal with downloading and uploading - especially if your List is extremely large. 

As a bonus, it mitigates the selection bias because it randomly picks people throughout your List - because if you cut the list in half in Excel and it happens to be sorted in some kind of chronological order (by default), one common mistake for example, is that the first half is the more recent profiles, and the bottom half is the less recent profiles.  So in some ways, the first half might be a better audience than the second half.  Using “sampling” makes it more randomized and can mitigate the selection bias.

Hope this helps!

Several good ideas in here thanks! I don’t know about Google being more stable than Excel haha but certainly worth a try. I assume I can save as CSV.

The unrecognized columns I discovered is due to bringing in lists from various platforms (for eg sponsoring a raffle on some random platform and then you get that list with totally random columns.) I will probably have to go through the 6,000 manually to get rid of those.

I thought about segments too.  Your suggestion is good. One reason I want to split the main list is to only un-suppress one list at a time so I’m not obligated to pay monster fees. (I only email like once a month so it’s a loss-making proposition to use Klaviyo)  So I’ll have to see if that works. One wrinkle is that you can’t line several sends up at once. (EG section a, b, c, of a list.) You have to wait till one is totally done sending and then activate and send the next segment or you will go over the limit number of profiles. 

I second ​@retention play around sampling from your master list if exporting, splitting and again importign not working for you